Contents

Overall Back Story

the sonar fence was built when the DI built the barracks and first discovered the others. the others have some way of controlling the smoke monster as shown by Ben and his secret doorway ( leading to the idea of other doorway around the island since there are underground tunnels around the island AND the temple ). the DI built the sonar fence as a way of protecting the barracks from the hostiles/smokey. the keypad on both sides was to ensure no one can get in/out of the barracks when leaving/entering the barracks. the sonar fence was built above another magnetic pocket within the island ( like the orchid and the swan) which creates the magnetic fence made by the pylons. for some reason, the smoke monster can not go through the magnetic field, There was the time in season 4 when the monster got through when ben summond it, however that was because Keamy got Alex to turn the fence off. finally, since they built the fence above the magnetic field they have a way of tapping into its power, so they can control the amount of magnetism is emitted allowing anyone on the island to get through or not get through depending on the level of lethality. also, in the barracks video the DI told the new recruits that it was there to protect wildlife so that they wont be scared or frightened of smokey and they are withholding info from the recruits. another example of withholding info is how Pierre Chang spoke of time travel to the construction site of the orchid while no one there knew what he was talking about [ kind of a weak statement compared to everything else.

the amount of portable sonar fence pylons Widmore had his men set up on Hydra Island (blocking passage to Widmore's submarine and the Ajira plane) is way too much to have possibly fit into his small submarine. This suggests that the pylons may have been originally used as portable substitutes during the construction of the 'solid' sonar fence on the main island and were later brought to a storehouse on Hydra Island.

Possible Mechanisms

The Producers confirmed (maybe someone can help me out to find the exact podcast) that the smoke monster is not able to separate itself from the ground and because of that it was not able to pass the fence

The use of sound to restrain an object, as seen during "Mikhail's death", would be nearly impossible without extensive technological and environmental considerations. It is also difficult to speculate how it could directly induce injury without nearly unreachable sound pressure levels. Such would also affect nearby individuals.

The effects during Mikhail's "death" could possibly be explained by electromagnetic effects, rather than sonic. A high field could possibly restrain a diamagnetic object (such as a person) as seen and appropriate oscillation of the magnetic flux could induce an injury. While the field strengths required would be exceptionally high, magnetism on that order of magnitude was observed in Season 2 episodes. Mikhail's reaction to the fence seemed to be from electrocution, because of his apparent difibrillation.

What's lethal for one person, may not necessarily be lethal for another. Although the fence did not kill Mikhail it is not to say it may not have been fatal to another person passing through it.

The barrier only affects certain individuals, those who have been implanted with a 'receiver' of some sort. Mikhail is one of these individuals.

The barrier is microwave rather than sonic in nature. A powerful microwave emitter will produce effects as seen when Mikhail was thrown into the field of effect, and is capable of inflicting gruesome, even fatal injury. However, on lower levels, the injuries, while severe, may not be fatal - merely incapacitating. Additionally, microwaves interfere with any metallic material, and especially unshielded electronics, which would explain why the Monster was unable to cross the perimeter. While a spectral analysis reveals a tonal that is very close to the resonant frequency of the human skull, this would affect only humans. Microwaves affect everything.

Although this theory is interesting, it has holes. Microwaves can cause severe burns, and a lot of bodily fluids including blood would come out.

Blood can be seen spurting out of Mikhail's ears before he collapses.

If the beam was powerful enough to cause Mikhail's blood to boil, I'm very sure he would be screaming due to the feeling of being on fire (Even though a microwave beam that strong would also cause your brain and a lot of your vital organs to shut down).

The perimeter fence initially creates a powerful barrier on either side of the person or object inside the barrier to prevent them from running through (the high pitched screeching heard when Mikhail was first pushed in) before emitting fatal doses of γ (Gamma) radiation sufficient to cause tissue death in the brain. The sound effect is used as a psychological deterrent (as the radiation would be invisible and the sound makes it appear that something is happening and it is not faked).

If the sonic fence was in fact a radiation fence, you would expect to see the grass and other vegetation near the fences to be dead, seeing as gamma rays don't discriminate on what they hit. It would also create a blast of gamma rays parallel to the fences perimeter every time it activated, meaning anyone who is unlucky enough to be standing on that line is going to have a real bad day.

The fence might be what made Ben grow up as another user stated. It also could not have been only for the wild life. It could have been put up to keep the island's original inhabitants away and in check.

I think its nonsense the fence being a time rift "like a bubble", with the island it think this theory is true but not the fence, also Richards aging is simple he was granted it by jacob or both jacob and Richard were granted imortality by someone else (the smoke monster maybe), youve got to admitt both jacob and Richard are quiet similar

Purpose

In "The Man Behind the Curtain", a character said that the fence was to keep out wildlife. Since the only dangerous wildlife we've seen is the polar bear (which we assume escaped from a zoo type station), it could very well be to keep out the Monster. If that's true, does that mean that The Hostiles controlled it, or could it have been there before they were as well?

The barrier creates a field similar to a no-chamber from Dune, and prevents the Others' remote viewing from seeing events inside of it. This is also why the Others' living and work spaces have so many cameras and microphones. The lethality of standing in the barrier is just a side effect.

The barrier was used to keep something or someone from escaping the Barracks. Perhaps DHARMA kept the "hostiles" inside the perimeter, and once "the incident" occurred, the hostiles took control of the Island. Of course, this would assume that the control panel would have been on the exterior face of a pillar rather than on the inside face to prevent attempts of escape. We still don't know the answer to this.

The spectral analysis of the sound made by the sonic barrier, one of the frequency bands corresponds to 972 Hz, which is the average resonant frequency of the human skull (and also 9x 108!). The barrier could be a means of not allowing the Monster to "get into the mind" or "skull" of it's target.

The barracks contains (or used to contain) the main control center for the entire island including control of Cerberus but due to 'the incident' Cerberus became damaged or wild and the sonic barrier was set up to protect the senior members of DHARMA on the Island.

Blast Door Map

May be represented by the red dotted line drawn around the "?" at the center of the map; as we now know that the alarm system completely surrounds the Barracks. The "?" mark might mean the the person who drew the blast door map didn't know what was inside the barrier.

This is unlikely considering that the "?" was revealed to be The Pearl, not The Barracks. Mr. Eko asked Locke to take him to the "?" which eventually led to the discovery of The Pearl.

Ghost Wall theory

In The economist, a deleted scene, shows Miles listens to the fence. We know/think that Miles can talk to and hear spirits (Confirmed Dead) by the way he speaks to the boy in the room, and Naomi. We also know that 'the monster', before killing Eko judged him (the camera went through the smoke, and his life could be seen), maybe the fence emits some - sort of life/death barrier, hence the reason why Miles can talk to it and why the monster can't past it.

This could also explain why Mikhail's ears bled when he was thrown through them. Perhaps the spirits yelled in his ears so loud only he could hear it.

Possible Origin of Smoke Monster Revealed

When watching "LaFleur", I noticed Horace ordering the Arrow to break out the "Heavy Ordinance" and make sure the fence was turned up to maximum. This made me wonder: if the Others had obviously forded the fence, then why have the fence turned up to maximum? Then, it hit me: he wants the fence turned up to max *because* they're breaking out the heavy ordinance: the Smoke Monster.
The fence was meant to stop it from backfiring on them.
The fact that an entire DHARMA station was devoted to R&D against the Others meant they were working on something new, a weapon unlike anything which could be transported to the island. That means they developed something entirely new, based on their research of the island. The Smoke Monster, which has been confirmed to be unable to leave the ground, could be a type of smoke-sheathed device that is controlled via an underground tunnel. This would mesh with the clanking, winching, metal-on-metal sound it makes, as well as Rose saying it sounds familiar to her (keep in mind she lived in New York City, crisscrossed by elevated subways on *tracks*).

I agree with this, it makes sence why else mention about the Heavy Ordinance if it served no purpose?

We have recently seen that The Monster "lives" in the tunnels under The Temple. Considering the hieroglyphics, I would hesitate to say that anyone, (except possibly Richard and Jacob) knows where it came from.

Good theory, but there are clear drawings of the smoke monster on the tunnel walls that obviously predate the DHARMA Initiative.

The Fence is a substitute of Ash Circles

The DHARMA initiative build the fence as a proxy of Ash Circles. Maybe they could investigate the composition of the Man in Black in his Monster form, and they created a scientific way to stop and control him. That's why the summon room is in Horace home. The initiative was fully aware of the tunnels and the summon chamber, and build the Barracks with the propose to research it. Horace could analyze in more detail the hieroglyphs and their meanings, only to establish the power of the monster and use it later as a weapon, that's why he is in charge of the Arrow Station. He knows the Monster is the enemy of the Hostiles (protected by Jacob), so the initiative tried to find a way to use the monster to destroy the hostiles and they keep with their research with no menaces in the future. The hieroglyphs of the chamber maybe details the ways to stop the monster and the use of ash circles. Due the fact that they did not have the ingredients of the Ash, They would to create a more complex system which works with the same accuracy to block the attacks of the Man in Black, in case they could not control him.

Incongruous Technology

We are told that the DHARMA Initiative built the Sonic Fence. But the level of technology involved seems too advanced for a group in the early 70's. The posts show a contemporary numeric keypad, yet other examples of keypads and keyboards we have seen used are old and clunky, and the phones are rotary dials. How did the DI in the early 70's get this technology? From the future?

They could have upgraded in the early 90s as tensions were running high just before the purge.

The Others put up new keypads because they had defeated the old ones and knew someone else might be able to as well.

Miscellaneous

Juliet says in Left Behind "We don't know what it is but we know it doesn't like our fences". This suggests that there is more than one sonic fence.

She could be referring to the fact that it are lots of pillars as opposed to whole sets of pillars.

Juliet mentioning this could mean that the fences were meant for another purpose, and the fact that it keeps out the monster is a side effect of it's real purpose.

The fence has different settings determined by an implant in the person who is in the way of the fence. When Mikhail broke the sensor beam there was a high pitched screeching not unlike the sound a 1960s X-Ray machine sounds, perhaps this determined that Mikhail was not an enemy but injured him enough so that he would heal. When the Monster broke them beam it instantly activated the waves which made it retreat suggesting that the barrier can detect what is near it